1. Field of the Invention
The present invention relates to amplifier technology, and more particularly to a differential amplifier which is thermally stable by adopting a ballast resistor to a differential structure of a unit transistor pair, such that the differential amplifier prevents heat effect phenomena, such as performance deterioration and device destruction by heating, and, at the same time, improves or maintains other performance traits, thereby achieving high gain, high efficiency, high linearity, and wide bandwidth characteristics, and a method for generating a computer simulation model of the differential amplifier.
2. Description of the Related Art
Generally, an amplifier is designed to obtain high output power and high efficiency. When high output power is an objective, a large amount of heat is inevitably generated inside an amplifier, such that the heat causes performance deterioration or destruction of unit transistors.
Especially, since bipolar transistors have a thermal characteristic of positive feedback, in a power amplifier designed by connecting a plurality of transistors, a large amount of heat is generated at the middle portion, in which transistors are more tightly packed, than at the edge. Also, since turn-on voltage at a relatively high temperature is lowered due to positive feedback and larger currents are generated in the middle of the power amplifier, a large amount of heat is generated therein.
As such a feedback is repeated, temperature of the transistors in the middle of the power amplifier is increased, and a large amount of current flows therethrough, such that the power amplifier suffers complete failure, finally resulting in complete destruction of the transistors.
In order to prevent such an outcome, each unit transistor forming an amplifier employs a ballast resistor to have negative feedback characteristics, such that thermal equilibrium can be maintained among unit cells, thereby protecting the transistors from destruction.
FIG. 1 is a view illustrating a circuit of an amplifier according to the prior art, in which each unit cell 15 includes a transistor 13 and a ballast resistor 14, which is connected to the emitter of the transistor 13, and thus forms thermally negative feedback. Therefore, the amplifier prevents heat-induced performance deterioration and device destruction.
However, since such a ballast resistor 14, employed in each cell to form thermally negative feedback, forms electrically negative feedback as well as thermally negative feedback, gain and efficiency of the amplifier are decreased.
FIG. 2 is a view illustrating a circuit of an amplifier according to another embodiment of the prior art, in which each unit cell 27 includes a unit transistor 24, a capacitor 26 connected to the base of the transistor 24 to transfer an RF signal to the base and a ballast resistor 25 located between a node, which is positioned between the capacitor 26 and the base, and DC voltage lead 22. Therefore, the amplifier prevents loss of an RF signal and is thermally stable through the DC bias path including the ballast resistor 25.
However, the prior art amplifier has disadvantages in that frequency bandwidth of the amplifier is decreased by such a capacitor, which is serially connected to the base to separate the RF signal path from the DC bias path, and also, its size is enlarged due to the capacitors.